Voltage-gated calcium channels are membrane-spanning, multi-subunit proteins that open in response to membrane depolarization, allowing Ca entry from the extracellular milieu. Calcium channels were initially classified based on the time and voltage-dependence of channel opening and on the sensitivity to pharmacological block. The categories were low-voltage activated (primarily T-type) and high-voltage activated (L, N, P, Q or R-type). This classification scheme was replaced by a nomenclature based upon the molecular subunit composition, as summarized in Table I (Hockerman G H, Peterson B Z, Johnson B D, Catterall W A. 1997. Annu Rev Pharmacol Toxicol 37: 361-96.)
There are four primary subunit types that make up calcium channels—α1, α2δ, β and γ (See, e.g., De Waard et al. Structural and functional diversity of voltage-activated calcium channels. In Ion Channels, (ed. T. Narahashi) 41-87, (Plenum Press, New York, 1996)). The α1 subunit is the primary determinant of the pharmacological properties and contains the channel pore and voltage sensor (Hockennan G H, Peterson B Z, Johnson B D, Catterall W A. 1997. Annu Rev Pharmacol Toxicol 37: 361-96; Striessnig J. 1999. Cell Physiol Biochem 9: 242-69). Ten isoforms of the α1 subunit are known, as indicated in Table I. The α2δ subunit consists of two disulfide linked subunits, α2, which is primarily extracellular and a transmembrane δ subunit. Four isoforms of α2δ are known, α2δ-1, α2δ-2, α2δ-3 and δ2δ-4. The β subunit is a non-glycosylated cytoplasmic protein that binds to the α1 subunit. Four isoforms are known, termed β1 to β4. The γ subunit is a transmembrane protein that has been biochemically isolated as a component of Cav1 and Cav2 channels. At least 8 isoforms are known (γ1 to γ8) [Kang M G, Campbell K P. 2003. J Biol Chem 278: 21315-8]. The nomenclature for voltage-gated calcium channels is based upon the content of the α1 subunit, as indicated in Table I. Each type of α1 subunit can associate with a variety of β, α2δ or γ subunits, so that each Cav type corresponds to many different combinations of subunits.
Cav Nomenclatureα1 subunitPharmacological nameCav1.1α1SL-typeCav1.2α1CL-typeCav1.3α1DL-typeCav1.4α1FCav2.1α1AP- or Q-typeCav2.2α1BN-typeCav2.3α1ER-typeCav3.1α1GT-typeCav3.2α1HT-typeCav3.3α1IT-type
Cav2 currents are found almost exclusively in the central and peripheral nervous system and in neuroendocrine cells and constitute the predominant forms of presynaptic voltage-gated calcium current. Presynaptic action potentials cause channel opening and neurotransmitter release is steeply dependent upon the subsequent calcium entry. Thus, Cav2 channels play a central role in mediating neurotransmitter release.
Cav2.1 and Cav2.2 contain high affinity binding sites for the peptide toxins ω-conotoxin-MVIIC and ω-conotoxin-GVIA, respectively, and these peptides have been used to determine the distribution and function of each channel type. Cav2.2 is highly expressed at the presynaptic nerve terminals of neurons from the dorsal root ganglion and neurons of lamina I and II of the dorsal horn (Westenbroek R E, Hoskins L, Catterall W A. 1998. J Neurosci 18: 6319-30; Cizkova D, Marsala J, Lukacova N, Marsala M, Jergova S, et al. 2002. Exp Brain Res 147: 456-63) Cav2.2 channels are also found in presynaptic terminals between second and third order interneurons in the spinal cord. Both sites of neurotransmission are very important in relaying pain information to the brain.
Pain can be roughly divided into three different types: acute, inflammatory, and neuropathic. Acute pain serves an important protective function in keeping the organism safe from stimuli that may produce tissue damage. Severe thermal, mechanical, or chemical inputs have the potential to cause severe damage to the organism if unheeded. Acute pain serves to quickly remove the individual from the damaging environment. Acute pain by its very nature generally is short lasting and intense. Inflammatory pain, on the other hand, may last for much longer periods of time and its intensity is more graded. Inflammation may occur for many reasons including tissue damage, autoimmune response, and pathogen invasion. Inflammatory pain is mediated by a variety of agents that are released during inflammation, including substance P, histamines, acid, prostaglandin, bradykinin, CGRP, cytokines, ATP, and other agents (Julius D, Basbaum A I. 2001. NATURE 413 (6852): 203-10). The third class of pain is neuropathic and involves nerve damage arising from nerve injury or viral infection and results in reorganization of neuronal proteins and circuits yielding a pathologic “sensitized” state that can produce chronic pain lasting for years. This type of pain provides little or no adaptive benefit and is particularly difficult to treat with existing therapies (Bridges D, Thompson S W N, Rice A S C (2001) Mechanisms of neuropathic pain. Br J Anaesth 87:12-26).
Pain, particularly neuropathic and intractable pain is a large unmet medical need. Millions of individuals suffer from severe pain that is not well controlled by current therapeutics. The current drugs used to treat pain include non-steroidal anti-inflammatory drugs (NSAIDs), cyclo-oxygenase 2 (COX-2) inhibitors, opioids, tricyclic antidepressants, and anticonvulsants. Neuropathic pain has been particularly difficult to treat as it does not respond well to opioids until high doses are reached. Gabapentin is currently the most widely used therapeutic for the treatment of neuropathic pain, although it works in only 60% of patients and has modest efficacy. The drug is generally safe, although sedation is an issue at higher doses.
Validation of Cav2.2 as a target for the treatment of neuropathic pain is provided by studies with ziconotide (also known as ω-conotoxin-MVIIA), a selective peptide blocker of this channel (Bowersox S S, Gadbois T, Singh T, Pettus M, Wang Y X, Luther R R. 1996. J Pharmacol Exp Ther 279: 1243-9; Jain K K. 2000. Exp. Opin. Invest. Drugs 9: 2403-10; Vanegas H, Schaible H. 2000. Pain 85: 9-18). In man, intrathecal infusion of Ziconotide is effective for the treatment of intractable pain, cancer pain, opioid resistant pain, and neuropathic pain. The toxin has an 85% success rate for the treatment of pain in humans with a greater potency than morphine. An orally available antagonist of Cav2.2 should have similar efficacy without the need for intrathecal infusion. Cav2.1 and Cav2.3 are also in neurons of nociceptive pathways and antagonists of these channels could be used to treat pain.
Antagonists of Cav2.1, Cav2.2 or Cav2.3 should also be useful for treating other pathologies of the central nervous system that apparently involve excessive calcium entry. Cerebral ischaemia and stroke are associated with excessive calcium entry due to depolarization of neurons. The Cav2.2 antagonist ziconotide is effective in reducing infarct size in a focal ischemia model using laboratory animals, suggesting that Cav2.2 antagonists could be used for the treatment of stroke. Likewise, reducing excessive calcium influx into neurons may be useful for the treatment of epilepsy, traumatic brain injury, Alzheimer's disease, multi-infarct dementia and other classes of dementia, amyotrophic lateral sclerosis, amnesia, or neuronal damage caused by poison or other toxic substances. The distribution of Cav2 channels in the central nervous system also suggests that antagonists are likely to provide efficacy against a number of psychiatric disorders, such as anxiety and depression and will also ameliorate insomnia.
Cav2.2 also mediates release of neurotransmitters from neurons of the sympathetic nervous system and antagonists could be used to treat cardiovascular diseases such as hypertension, cardiac arrhythmias, angina pectoris, myocardial infarction, and congestive heart failure. Inhibition of neurotransmission in the gut can be useful for treating irritable bowel disease and other gastrointestinal disorders.
Block of Cav3 channels can also result in therapeutically useful effects. Low-voltage activated calcium currents associated with these channels were first described in dorsal root ganglia neurons and suppression of Cav3.2 with antisense provides efficacy in the chronic constriction model for neuropathic pain (Bourinet E, Alloui A, Monteil A, re C B, Couette B, Poirot O, Pages A, McRory J, Snutch T P, Eschalier A, Nargeot J I (2005) Silencing of the Cav3.2 T-type calcium channel gene in sensory neurons demonstrates its major role in nociception. EMBO Journal 24:315-324). Block of Cav3 channels is also likely to result in efficacy against inflammatory pain and acute pain. Current through these channels supports pacemaker electrical activity in the central nervous system, and antagonists are likey to have activity as anti-convulsant agents. Unlike Cav2 channels, Cav3 channels are widely distributed in vascular smooth muscle and myocardial cells. Cav3 antagonists may thus have utility against hypertension, angina and arrhythmias.
Cav2 and Cav3 channels are also found in a variety of endocrine cells and antagonists may be useful for the treatment of endocrine disorders such as diabetes.